This invention relates to a method of producing a solid-state imaging device, and more particularly to a method of manufacturing color filters for use in a solid-state imaging device.
In recent years, solid-state imaging devices have been energetically developed as imaging devices of the coming generation to replace image pickup tubes or electron tubes (described in detail in "Electronics," April 1976, pp. 368-372; "Journal of the Society of Television," Vol. 33, No. 7 (1979), pp. 548-566; etc.). The colorization of the solid-state imaging devices has been made by forming color filters on a photoelectric conversion region. The color filters are classified into an organic filter obtained by dyeing an organic material such as gelatin, and an inorganic filter exploiting the optical interference. From the viewpoint of low cost, however, the organic filter of the former is used in case of the electron tube already employed in the television broadcast. Further, the organic filter is indispensable in case of the solid-state imaging device.
The inventors have found out that, as compared with an optically flat glass plate serving as the substrate of the imaging electron tube, a solid-state imager LSI (Large Scale Integration) serving as a substrate to form the color filters thereon in the production of the color solid-state imaging device involves very inconvenient problems in stacking or depositing the color filters as will be described later.
First, the features of the solid-state imaging device will be described.
(i) Since the solid-state imager LSI is produced with IC manufacturing techniques (both a MOS type imager LSI and a CCD type imager LSI ae produced with the MOS-IC technology), it has an unevenness of 1.5-2.5 .mu.m due to an insulating film and a conductive film for interconnection formed on a silicon substrate.
(ii) A metal of high conductivity (for example, Al or Mo) is used for the interconnection, but the metal exhibits a high reflectivity.
(iii) In order to raise the resolution, the size of one picture element is reduced with enhancement in the fabrication techniques. In case of resorting to the recent 3 .mu.m-rule technique, the size is as small as approximately 20 .mu.m, and in case of resorting to the 1 .mu.m-rule techniques in the future, it is anticipated to be reduced down to approximately 10 .mu.m.
(iv) In case of the electron tube, no index signal (synchronized signal for reading out color signals) is possessed, and hence, the configuration format of the filters is the stripe filter configuration. In contrast, in case of the solid-state imaging device, clock pulses for driving the device can be utilized for index signals, and hence, the mosaic filter configuration (tiling filter configuration) which can attain a high resolution with a small number of picture elements is used (of course, the stripe filter configuration is also usable in the solid-state device, but by way of example, when red, blue and green being the three primary colors are repeated with stripes, the color resolution degrades to 1/3, so that the stripe filter configuration is not used usually).
The color filters are formed successively for the respective colors in such a way that a filter material is placed on the solid-state imager LSI having been finished up to the interconnections of a photoelectric conversion element array, scanning circuitry, etc., and that the filter material is left on the photoelectric conversion elements through a photoetching process (exposure and developing) similar to the fabrication technique of the underlying LSI, whereupon the left parts are dyed in a predetermined color (in case of the three primary colors, three fabrication steps are required). Here, the foregoing four items form causes for the following problems:
The unevenness stated in (i) causes light to scatter in an area between the convex and concave parts at the exposure, so that the filter becomes narrower or conversely wider than a predetermined region decided by a photomask. The problem of the reflection stated in (ii), similarly causes the scattering of the exposure and gives rise to the narrowing or widening of the filter. Item (iii) causes the neighboring effect of light (a kind of interference) at the exposure, and the light penetrates to a region prevented from being exposed thereto by the photomask, so that the filter becomes wide. Further, since the filters have patterns in the mosaic (tiling) configuration owing to Item (iv), the etching amount is not uniform but differs depending upon positions. In addition, on account of the neighboring effect stated above, nonuniformity in the etching amount having a positional periodicity arises depending upon the configuration of the filters. That is, the narrowing and widening deviate depending upon positions, and an obtuse unevenness appears within each pattern. Among these problems, especially the widening causes the overlap between the filters of the different colors or the phenomenon of hue mixing, which is a serious problem of degrading the picture quality.